Write the words 'Will you marry me?' in the sand. You can write it before you bring her or write it while she's lounging in the sun. When she finds it, get down on one knee with the ring and ask her to marry you.
Build her a sandcastle at the beach together, then place the ring on the highest tower while she's not looking. When she finds it, tells her that she's your princess and then ask for her hand in marriage.
Fly a kite with the words "Will you marry me?" on it. Tell her that your love for her soars higher than any kite and then pop the question.
Spell it out in seashells on the beach. Save the shells from the proposal and use them in a beach themed wedding.
Take a paddle boat out on the lake and propose as you drift along drinking wine and holding each other close.
Tell her you see something at the bottom of the pool, dive in, and come up with her ring! When she asks what you have, put it on her finger and ask for her hand in marriage.
Write it on the back of a sand dollar in water proof permanent ink. When you suddenly "find" it in the ocean, give the proposal to her along with the ring. (Note, you'll need to use a store bought sand dollar for this.)
Put the ring in an oyster shell. She'll be surprised to find a diamond instead of a pearl.
Take the engagement ring with the ring box and place it in some kind of well-made outer box. Take the outer box, with the ring box inside, and bury it in a romantic spot on the beach. Mark the spot with something so that you don't lose track of it. Bring your girlfriend to the spot and begin building a sand castle together. All of the sudden, as you are digging in the sand, tell her that you feel something hard. Have her help you dig it out and when the box is revealed, have her open it. When she opens it, get down on one knee and ask her to marry you.
Bend down to pick up a shell and get down on one knee. Instead of handing her the shell, hand her the ring and ask her to marry you.
Canoe down the river and then drift along as you sit close and share your hopes and dreams. Propose as you hold her close.
Take her to the beach and present her with a hand painted coconut with your proposal on it. Carry the theme a step further and honeymoon in the tropics!